The PC behind
fa0/1 would send a DHCPDISCOVER broadcast on the subnet it belongs to, to discover the DHCP server. In order for the
fa0/3 to reach the DHCP server, a DHCP relay agent would have to be configured.
In small networks, where only one IP subnet is being managed, DHCP clients communicate directly with DHCP servers. However, DHCP servers can also provide IP addresses for multiple subnets. In this case, a DHCP client that has not yet acquired an IP address cannot communicate directly with the DHCP server using IP routing, because it does not have a routable IP address, does not know the link layer address of a router and does not know the IP address of the DHCP server.
In order to allow DHCP clients on subnets not directly served by DHCP servers to communicate with DHCP servers, DHCP relay agents can be installed on these subnets. The DHCP client broadcasts on the local link; the relay agent receives the broadcast and transmits it to one or more DHCP servers using unicast. The relay agent stores its own IP address in field GIADDR field of the DHCP packet. The DHCP server uses the GIADDR-value to determine the subnet on which the relay agent received the broadcast, and allocates an IP address on that subnet. When the DHCP server replies to the client, it sends the reply to the GIADDR-address, again using unicast. The relay agent then retransmits the response on the local network.
In this situation, the communication between the relay agent and the DHCP server typically uses both a source and destination UDP port of 67. Source